Inheritance of a mutation at the Rb-1 locus, which has been mapped to band q14 of human chromosome 13, results in predisposition to retinoblastoma. Cloned DNA segments which have homology to arbitrary loci on human chromosome 13 and which reveal polymorphic restriction endonuclease recognition sequences will be isolated. These segments will be used as molecular probes in a comparison of constitutional and tumor genotypes in order to look for somatic genetic events that might occur during tumorigenesis. Similarly, constitutional and tumor genotypes of patients affected by secondary malignancies will be determined and the frequency with which heterozygosity is lost in these tumors will be ascertained. Loci homologous to the cloned DNA segments will be physically mapped through the use of various somatic cell hybrids and by in situ hybridization to metaphase chromosomes. This physical map will be compared to a genetic linkage map which will be obtained by segregation analysis in human pedigrees. These studies should provide basic information about chromosomal mechanisms involved in human tumor formation in vivo and about the frequency of recombination between many loci on this autosome. If these mechanisms are determined to be involved in other tumor types, a general methodology for mapping genes which predispose to tumorigenesis will be obtained, facilitating their identification, isolation and molecular analysis. Additionally, genetic markers should be identified which will be useful predictive tools in genetic counseling of parents afflicted with retinoblastoma.